1. Technical Field of the Invention
The present invention generally relates to a cellular telephone, and more specifically, relates to a cellular telephone used in Code Division Multiple Access (CDMA) system.
2. Description of Related Art
An example of a CDMA cellular telephone system is disclosed in U.S. Pat. No. 5,056,109. In this example, a cellular telephone has transmit power control circuitries 76 and 80 connected in series between a transmit modulator 84 and an antenna 70 and controlled by a control processor 78, so as to control and adjust a power level of transmission power. The control and the adjustment of the power level are required so as to overcome deleterious terrestrial channel fading between a terrestrial base station (a cell-site station) and a mobile unit. The terrestrial channel fading is caused by a path loss in the terrestrial channel, which is on the order of over 80 dB, and the power adjustment at the mobile unit is implemented by a predetermined amount, nominally 1 dB.
The typical standard for the CDMA cellular telephone system enacted in the U.S. is TIA IS-95 (hereinafter IS-95). An example of a transmitter applying IS-95 is shown in FIG. 11. In this example, a modulated signal converted into a transmitting frequency band is supplied to a variable amplitude amplifier 230 and the output of the variable amplitude amplifier 230 is further amplified by a power amplifier means 250 and transmitted to an antenna 450 through a duplexer 400. A controller 380 supplies a gain control signal to the variable amplitude amplifier 230 in order to adjust the gain such that the power transmitted from the antenna 450 will satisfy the required value of transmitting power. The gain control signal is also supplied to a level detecting means 390. The level detecting means 390 supplies a bias signal to the power amplifier means 250 for adjusting the transmitting power. The level detecting means 390 detects a level of the gain control signal, and as shown in FIG. 12, when the level is high (e.g. Gn), it outputs bias value of B2. Then the level decreases, and the level crosses a threshold value, the level detecting means 390 changes the bias abruptly from B2 to B1. Current of the power amplifier means change abruptly when the gain level crosses the threshold value. In IS-95, open-loop power control and closed-loop power control are employed in order to regulate a receiving power at the cell-site station. The open-loop power control, which definitely determines a transmitting power according to an information indicating an intensity of electric field detected by a receiver, does not require a severe accuracy (generally within the range of 9.5 dB). On the other hand, the closed-loop power control performs fine control according to an information indicating a variation of a gain of the signal transmitted from the cell-site station (generally 1 dB step). In this case, the transmitter performs the open-loop power control at first, then it performs the closed-loop power control for the transmitted power to converge into a desired value which the cell-site station requires.
Japanese Patent Laid-open No. Hei-9-46152 discloses a mobile unit including a receiver and a transmitter, where the transmitter has a variable amplitude amplifier for adjusting the power level of the transmission power. In this related art, the mobile unit includes a high frequency power amplifier having a field effect transistor (FET) as an amplifying element, and the variable amplitude amplifier is composed so as to control a bias condition of the FET according to a transmission power. Amount of attenuation according to the transmission power, and gate voltage of the FET corresponding to the amount of attenuation are controlled by degrees, i.e. by 4 dB, according to the predetermined values. When the transmission power is low, the gate voltage is reduced so as to reduce current consumption for prolonging battery life.
A problem in the related art is that the large current consumption shortens the battery life, which shortens the hours of communication on the phone.
Another problem in the related art is that the compensation of the battery life requires an increase of volume and weight of the battery. Further related art is also disclosed in Japanese Patent Application Laid-open Nos. Hei-04-277909 and Hei-06-252797.